Woolmer and McCulloch [1] describe the topology of a Y machine, discussing its advantages of reduced iron in the stator enabling an improvement in torque density. It comprises a series of coils wound around bars spaced circumferentially around the stator, ideally axially disposed, (ie parallel the rotation axis of the rotor). The rotor has two stages comprising discs provided with permanent magnets that face either end of each coil of the stator. The magnetic path at any stage of operation is: through a first coil into a first magnet on a first stage of the rotor; across a back iron of the rotor to an adjacent second magnet on the first stage; through a second coil of the stator adjacent the first coil; into a first magnet on the second stage of the rotor aligned with the second magnet on the first stage; across the back iron of the second stage to a second magnet on the second stage and aligned with the first magnet on the first stage; and completing the circuit through the first coil.
One difficulty with electric machines generally is to provide adequate cooling. This is a particular problem with a Y machine having a high torque density that significant heat is generated in the coils at high torques and is often a limiting factor in the torques that can be employed, at least for extended periods of time. Also, the coils are isolated from one another and therefore cooling only one region of the motor is insufficient as there is low conduction of heat between coils.
WO-A-2006/066740 discloses a Y machine comprising a housing having a cylindrical sleeve mounting stator coils internally, the sleeve being hollow whereby cooling medium is circulated. However, the coils are embedded in a thermally conducting material to carry heat to stator housing. A rotor is rotatably journalled in the housing. The stator bars appear to be laminated, as they are in GB-A-2379093 that also discloses a Y machine, as does WO-A-03/094327. No cooling arrangements are mentioned.
U.S. Pat. No. 6,720,688 discloses a Y machine in which the rotor acts as a vane pump to circulate fluid within a chamber defined by a stator housing through which a rotor shaft, supported on bearings in the housing and carrying the rotor, extends. The fluid cools stator coils.
Of course, cooling problems are not limited to Y motors.
The idea of evaporative cooling has been employed in, for example, SU-955379, where a hollow rotor shaft appears to extend into an external rotating housing so that refrigerant evaporating in the shaft cools the rotor and vapour condenses in the external housing releasing its heat before returning as a liquid. U.S. Pat. No. 5,394,040 discloses a similar arrangement. SE-A-7411152 likewise appears to disclose evaporative cooling of a motor. These devices are passive, where the cooling circuit is self-driven, but more active arrangements are disclosed. U.S. Pat. No. 3,217,193 sprays liquid refrigerant on the hot parts of a motor or generator. US-A-2007/0199339 discloses a complete refrigerant circuit with valves ensuring correct directional flow between passages through the stator and an external heat exchanger. This development is of particular interest to the present invention which finds a primary application in wheel motors for vehicles.
Indeed, it is an object of the present invention to provide an electric machine with effective, but nevertheless passive, evaporative cooling arrangements. It is a particular object to minimize the additional elements and equipment required for cooling, whereby the cost in terms of power lost through the cooling arrangements can be minimized.